Vacuum container



Sept. 22, 1964 R. A. BERG-AN VACUUM CONTAINER Original Filed Aug. 23,1956 FIG.|

INVENTOR. RA. BERGAN FIG. 2

ATTORNEY detectors. 'the use of these devices.

United States Patent 3,149,741 VACUUM CONTAINER Reuben Allard Bergen,Houston, Tex., assignor, by mesne assignments, to Dresser Industries,Inc., Dallas, Tex, a corporation of Delaware Original application Aug.23, 1956, Ser. No. 605,903, new Patent No. 3,055,101, dated Sept. 25,1962. Divided and this application July 13, 1961, Ser. No. 123,888

2 Claims. (Cl. 220-) This application is a division of my copendingapplication Serial Number 605,903, filed August 23, 1956, now Patent No.3,055,101, for Vacuum Container and Method of Processing Same.

This invention relates to static vacuum insulating containers and isparticularly directed to novel vacuum containers for maintainingradioactivity detectors at substantially constant low temperaturesduring borehole logging operations in hot wells.

In the art of radioactivity well logging, measurements are made ofradioactivity occurring in the formations surrounding a well to obtainvarious geophysical information. The instruments used for making suchmeasurements often employ scintillation phosphors as radiation However,numerous problems are presentedby The instruments in which the detectorsare housed must 'be suspended by a cable and lowered many thousands offeet down a well. During such operations, the instrument frequentlybumps against the sides of the well and may be subjected to temperaturesas high as 400 F. Un-

fortunately, scintillation phosphors and some of the electronicequipment required, such as photomultiplier tubes, are sensitive totemperature changes and operate most effectively at constanttemperature, preferably at 30 to 50 F. Thermal insulation is required,but there is very ally desirable to employ some form of refrigeration.

It has long been known that double walled containers are extremelyuseful for maintaining articles contained therein at low temperatures inatmospheres of higher temperatures, and it has been found heretoforethat these devices could be improved considerably by evacuating thespace between the walls of the container, as in Dewar flasks. Byinserting a shield which will reflect thermal radiation between thewalls, containers have been made which are satisfactory at moderatetemperatures. However, it has been found that substantially allmaterials contain some gases, such as carbon dioxide, carbon monoxide,oxygen and nitrogen, and when the space between the walls of a containeris highly evacuated, these gases tend to evolve from the material of thewalls and degrade the vacuum, thus reducing the thermal insulatingproperties of the container. To overcome this, it is common to insertinto the space between the walls a small quantity of charcoal whichreadily adsorb any gases evolved. Unfortunately, charcoal is effectiveonly at extremely low temperatures, such as those obtained withrefrigerants like liquid air, and flasks so made are used for containingliquid air when the ambient temperature is atmospheric.

However, these refrigerants are very expensive and are not suitable foruse in cooling well logging instruments, especially when the ambienttemperature of the well approaches 400 F. Consequently, even tie bestmetal vacuum containers of the prior art have been useful only atatmospheric temperatures. Above this limit, containers of glass, whichmay be more easily degassed, have between the walls of the container.

"ice

been employed but these are much too fragile to withstand the roughtreatment which well logging instruments must encounter. Moreover, Pyrextype glass, which will withstand prolonged exposure to hightemperatures, have a high boron content. Boron has a high capture crosssection for neutrons and boron flasks would interfere with themeasurement of neutrons in radioactivity well logging.

The disadvantages of prior art vacuum containers are overcome with thepresent invention and novel methods of constructing metal vacuumcontainers are provided which tremendously increase the life and usefulrange of such devices. Thus, with the present invention, it is possibleto construct metal vacuum containers which, employing only 300 grams ofice as a refrigerant, will keep the contents at temperatures of 30 to 50F. for logging cycles as long as 10 hours, even at well temperatures of300 to 400 F. Furthermore, the containers of the present invention arephysically rugged and can undergo extremely rough treatment withoutdeveloping leaks.

These advantages of the present invention are preferably attained byforming a double walled container of metal having a low thermalconductivity and a low emissivity and coating the inner walls of thecontainer with an inorganic material which has an emissivity lower thanthat of the container and which is readily capable of adsorption of, orchemical combination with most of the gases normally occurring asimpurities in metals. This is preferably accomplished by forming athermally reflective shield of a metal having the desired properties andhaving a sufiiciently high vapor pressure at degassing temperatures tocause deposition of the metal. The shield is preferably assembled in thespace between the walls of the container and the assembly is thendegassed by simultaneously heating the assembly and evacuating the spaceThe deposited metal will serve as a getter to adsorb or combine with anyevolved gas within the container, thus assuring long life and highlyefficient thermal insulation of the container.

Accordingly, it is an object of the present invention to provide a novelvacuum container which is capable of sustained use at high temperatures.

Another object of the present invention is to provide a novel vacuumcontainer which is mechanically rugged.

A further object of the present invention is to provide a novel meansfor maintaining high vacuum between the walls of vacuum containers forlong periods of time and even at high temperatures.

A specific object of the present invention is to provide a novel vacuumcontainer formed of metal having a low thermal conductivity and a lowemissivity wherein the inner walls of the container are coated with aninorganic material which has an emissivity lower than that of thecontainer material and which is readily capable of adsorption andchemical combination with gases.

Another specific object of the present invention is to provide a novelmethod of constructing vacuum containers comprising forming a doublewalled container of metal having a low thermal conductivity and a lowemissivity, forming a shield of an inorganic material which has anemissivity lower than that of the container material and which isreadily capable of adsorption of or chemical combination with gases,assembling the shield in the space between the walls of the container,and degassing the assembly by simultaneously heating and evacuating thespace between the walls of the container. 0

These and other objects and features of the present invention will beapparent from the following description wherein reference is made to thefigure of the accompanying drawing.

In the drawing:

FIGURE 1 is a vertical section through a typical vacuum containerembodying the present invention; and

FIGURE 2 is a sectional View of a portion of a modified vacuum containerembodying the invention.

In that form of the invention chosen for purposes of illustration in thedrawing, FIGURE 1 shows a double walled vacuum container 2 having anouter wall 4 and an inner wall 6 joined at the upper ends 8 thereof andhaving a space 10 therebetween. Preferably, Walls 4 and 6 arecylindrical and have closure members 5 and 7 respectively closing thefree ends thereof. If desired, the closure members may be integral withthe walls. The walls 4- and 6 are preferably formed of metal having alow thermal conductivity and a low emissivity. Stainless steel having aminimum carbon content has been found to have these characteristics andis particularly well suited to use in well logging operations as it isstrong enough to Withstand the severe shocks encountered when theinstrument strikes the wall of the well. In view of the high vacuumwhich must be maintained between the walls 4 and 6, it is preferablethat all joints, such as that at S, be Welded with an inert-gas-shieldedarc.

To reduce radiant heat transfer, a thermally reflective shield 12 isprovided formed of an inorganic material which has an emissivity lowerthan that of the container 2 and which is readily capable of adsorptionof or chemical combination with. most of the gases normally occurring inmetals. Copper is characteristic of such materials and is especiallysuitable as it is impervious to nitrogen, which is a major impurity ofstainless steel. The emissivity of the various metals may be found instandard handbooks, such as Handbook of Chemistry and Physics, ChemicalRubber Publishing Company, 33rd Edition (1951-1952), page 2455. Theshield 12 is positioned in the space 111 between the Walls 4 and 6 ofthe container 2 and is secured to one wall of the container 2, in thisinstance, inner wall 6, in any suitable manner.

As seen in FIGURE 1, the shield 12 is mounted by suitable means, such aa spolred member or spider 13, formed of low conductivity inorganicmaterial so as to provide a long path of low conductivity between theshield 12 and the wall 6 to which it is secured. In radioactivity welllogging instruments where the lateral dimensions are restricted topermit passage of the instrument through a borehole, it may bepreferable to secure the hub 15 to one of the closure members 5 and 7,which may be spaced farther apart than the cylindrical parts of walls 4and 6. As a matter of convenience, the shield 12 is secured, as shown inFIGURE 1, to the closure member 5. However, to minimize conductionlosses, the shield 12 should be secured to the wall having the leasttemperature differential with respect to the shield 12. If desired,stabilizing means, such as bumpers 14, formed of low conductivityinorganic material may be provided to position the free end of the innerwall 6 with respect to the outer wall. The bumpers 14 may be looselymounted in windows 17 in the shield 12 similar to the mounting of rollerhearings in a race. The bumpers 14 should be of slightly less diameterthan the distance between the wall 4 and 6. In this Way, the bumperswill have only intermittent contact with either wall and thermaltransfer will be minimized. Centering means, such as dimples 16, may beemployed to prevent the free end of shield 12 from contacting the outerwall 4 of the container. The dimples 16 are preferably small so thatwhen those on one side, for instance, dimple 19, are in contact With theWall 6, those on the other side, such as dimple 21, will not contact thewall 6. However, the dimples should be large enough to prevent theshield 12 from engaging the wall 4 at any time. Preferably, the dimples16 will be formed adjacent the upper end of the shield 12 and willengage the inner wall 6 at approximately the point of minimumtemperature differential along the conduction path between inner wall 6and outer wall 4.

After the shield 12 has been assembled in the container 2, a vacuum pumpsystem equipped with suitable pressure gauges is connected to theexhaust tube 13 and the space it) is evacuated to a desired pressure. Inthe case of a stainless steel container having a copper shield, thiswill be about 10- mm. of mercury as indicated by a gauge at the mouth ofthe exhaust tube 18. At the same time, the container 2 is heatedgradually until most of the impurity gases have been driven out of thevarious materials. During this procedure, the temperature and pressuremust be carefully controlled to prevent damaging the surfaces of theWalls 4 and 6 and the shield 12. For a stainless steel container havinga copper shield, this require heating the assembly to a temperature of900 to 1400 F. while never allowing the pressure at the exhaust tube 18to exceed 10 mm. of mercury. Approximately 12 to 24 hours are requiredfor this operation.

At the degassing temperature, several advantageous effects occur. In thefirst place, most of the impurity gases in the materials of the walls 4-and 6 and shield 12 will be driven out and will be pumped out or" theexhaust tube 18 by the vacuum pump system. In addition, at anytemperature above absolute zero (-460 F.), the molecules of any materialconstantly undergo thermal agitation and some of the molecules at ornear the surface or" the material will escape into the atmosphere. Thequantity of free molecules can be determined and is referred to as thevapor pressure of the material. As the temperature of the materialrises, the thermal agitation and, consequently, the vapor pressure alsorises. At degassing temperatures, the vapor pressure of any material isconsiderable. Thus, copper at 1270" F. has a vapor pressure of the orderof 10- mm. of mercury. Consequently, there are a tremendous number ofcopper molecules flying about within the space 19. The vapor pressure ofvarious metals at various temperatures is given in the Review ofScientific Instruments, Vol. 19, pages 920-922 (1948).

After the degassing temperature has been reached, the heat is removedand the assembly is allowed to cool. However, evacuation is continuedduring the cooling. Since the shield 12 is separated from the walls 4and 6 of the container and the space 19 is evacuated, the walls 4 and 6will cool much more rapidly than the shield 12. As the walls 4 and 6coo-l, the free molecules of the shield material will adhere to thesewalls and will form a permanent visible coating 20 on the walls 4 and 6.

As stated previously, evacuation of the space 10 is continued after theheat has been removed and, as the assem bly cools, the pressure withinthe space 11} will be reduced. When the pressure at the gauge on exhausttube 18 indicates approximately 10 mm. of mercury, the tube 18 is closedoff, as seen at 22, in any suitable manner and sealed, as by solder cap24.

The vacuum container is then ready for use and the coating 2% on thewalls 4 and 6 contributes greatly to the effectiveness of the device.The coating 2%} and the surfaces of the shield 12 will be smooth andshiny and, therefore, will tend to prevent passage of radiant heat.Moreover, the coating 25 and the surface of the shield 12 will be highlyactive chemically and will readily adsorb or combine with any gasmolecule which strike it. This action of the shield material willcontinue at some finite rate even after the container has been sealed,thereby maintaining the vacuum in space 10 and greatly extending theuseful life of the container. In addition, with a stainless steelcontainer having a copper shield, the copper coating on the Walls 4 and6 will be impervious to nitrogen, which is a major impurity in stainlesssteel. Consequently, the useful life of the container will be increasedstill further by preventing nitrogen in the steel from entering theevacuated space.

It has been found in practice that a container constructed in accordancewith the present invention, provided with a suitable insulating closure,is capable of maintaining an inside to outside temperature differentialof several hundred degrees for very long periods of time using only asmall amount of refrigerant. Thus, such containers used in radioactivitywell logging equipment and employing only about 300 grams of ice havebeen able to maintain an inside temperature of 30 to 50 F. throughoutlogging cycles of 8 to hours even in wells having temperatures over 300F.

A further advantage of the present invention resides in the fact thatwhen, eventually, it becomes necessary to re-evacuate the space 10, theaction of the shield material may be renewed by simply reheating thecontainer to the degassing temperature during the reevacua-tionoperation.

In some instances, it may be necessary or desirable to provide a moreefiicient getter to further extend the life of the container or topermit rejuvenation of the vacuum in space 10 without re-processing theentire container. This may readily be accomplished by employing themodified container disclosed in FIGURE 2.

FIGURE 2 shows a portion of a vacuum container 2 which is substantiallyidentical with that of FIGURE 1 but which has an air tight housing 26mounted thereon in any suitable location. An opening 28 is formed in theouter wall 6 of the container 2 and connects the interior of the housing26 with the space 10 inside the container 2. Within the housing 26,anampule 30 is provided containing a getter material which preferablyhas a high vapor pressure at the maximum operating temperatures and hasa relatively high mobility. The getter material should, obviously, behighly active chemically. Thus sodium has been found to be quite good.

The ampule 30 may be formed of substantially any inorganic material.However, the getter material must be free of impurities and the ampule30 must be loaded and sealed in a vacuum. Furthermore, the ampule 30must be able to withstand the processing operations, described above inconnection with FIGURE 1, without releasing the getter material.Subsequently, when the gettering action is desired, the ampule may bebroken either mechanically, for example, by crushing the housing 26, orthermally as by localized heating, to release the getter material. Thegetter material will then form a coating on the walls 4 and 6 and thesurfaces of the shield 12 in substantially the same manner as describedabove for the coating 20. If necessary or desirable, the housing 26 maybe locally heated to vaporize the getter material and to drive itthrough opening 28 into space 10. The material Will then getter any gaswhich has evolved into the space It) and the high vacuum will berestored. Like the shield material, the action of the getter may berenewed when necessary by heating the container sufiiciently to vaporizethe getter material.

If desired, the getter material may be released into space 10 of thecontainer during the original processing of the container, instead ofstoring it in housing 26. Moreover, other means of storing andintroducing the getter material may be employed. Numerous additionalvariations and modifications may also, obviously, be made withoutdeparting from the invention. Accordingly, it should be clearlyunderstood that those forms of the invention described above and shownin the figures of the accompanying drawings are illustrative only andare not intended to limit the scope of the invention.

I claim:

1. A double walled vacuum container comprising inner and outer Wallsspaced apart and formed of metal having relatively low thermalconductivity and relatively low emissivity, the space between said wallsbeing evacuated, a thermally reflective shield formed of gas sorptivemetal having an emissivity less than that of said walls mounted betweensaid Walls within said space, and a gas sorptive and chemically activecoating formed of the same material as said shield deposited on thefacing surfaces of said walls.

2. A double walled vacuum container comprising inner and outer wallsspaced apart and formed of stainless steel having minimum carboncontent, the space between said walls being evacuated, a thermallyreflective shield formed of gas sorptive copper mounted between saidwalls within said space, and a gas sorptive and chemically activecoating of copper deposited on the facing surfaces of said walls.

References Cited in the file of this patent UNITED STATES PATENTS1,204,838 Bartlett Nov. 14, 1916 1,315,624 Fahnestock Sept. 9, 19191,712,370 White May 7, 1929 1,738,991 Fink Dec. 10, 1929 2,547,607Sulfrian Apr. 3, 1951 2,759,134 Sullivan Aug. 14, 1956 FOREIGN PATENTS330,342 Great Britain June 12, 1930

1. A DOUBLE WALLED VACUUM CONTAINER COMPRISING INNER AND OUTER WALLSSPACED APART AND FORMED OF METAL HAVING RELATIVELY LOW THERMALCONDUCTIVITY AND RELATIVELY LOW EMISSIVITY, THE SPACE BETWEEN SAID WALLSBEING EVACUATED, A THERMALLY REFLECTIVE SHIELD FORMED OF GAS SORPTIVEMETAL HAVING AN EMISSIVITY LESS THAN THAT OF SAID WALLS MOUNTED BETWEENSAID WALLS WITHIN SAID SPACE, AND A GAS SORPTIVE AND CHEMICALLY ACTIVECOATING FORMED OF THE SAME MATERIAL AS SAID SHIELD DEPOSITED ON THEFACING SURFACES OF SAID WALLS.